Methanol is formed as a by-product of the kraft pulping process, when the hydroxyl on reacts with a lignin methoxyl group:lignin.OCH3+OH−→CH3OH+lignin.O−
Depending on the mill configuration, up to 90% of the methanol generated in the digester can be captured in the foul condensate from the digester and evaporator areas. The foul condensate is typically treated in a steam stripping system, where up to 95% of the methanol can be removed from the foul condensate and captured in the overhead vapours from the stripping process. The concentrated gas stream is often referred to as stripper off gas (SOG).
The SOG is then usually disposed of through thermal oxidation in a lime kiln, power boiler, recovery boiler, or dedicated incinerator. The SOG typically consists of about 40 to 70 wt % methanol, 5 to 10 wt % non-condensable materials, including sulphur compounds, and the balance water vapour.
Waste SOG can be burned as a replacement for fossil fuels. However, the value of SOG as a fuel depends on the amount of water vapour that it contains. Natural gas provides 50.5 MJ/kg (37.2 MJ/m3) heat of combustion, pure methanol provides 22.7 MJ/kg, and SOG containing 70 wt % methanol provides the equivalent of about 21.9 MJ/kg. The SOG provides less heat because the entrained water vapour must first be heated up to combustion temperature.
Chlorine dioxide is used in the pulp bleaching process; grade AA methanol (99.85 wt %) is used to manufacture ClO.sub.2. In a well-run mill, a methanol purification system would preferably be able to produce sufficient amounts of purified methanol for the demands of the ClO.sub.2 process, as well as some purified methanol for external sale. If a substantial portion of the methanol in the SOG can be recovered and purified to an industrial grade AA product, the methanol produced in a typical kraft pulping process could be worth as much as four and a half times more as a commodity than as a fuel.
There are numerous methanol purification systems in operation. Most such systems use some form of distillation to separate methanol from other compounds. See for example, U.S. Pat. No. 5,718,810 to Robbins and U.S. Pat. No. 6,217,711 to Ryham et al. Canadian Patent No. 1,0888,957 to Suokas et al., uses a combination of distillation steps and acid or alkaline oxidating treatments to separate the various fractions. Distillation separates the components of a solution by partial vapourization of the mixture and separate recovery of vapour and residual liquid. The more volatile constituents of the original mixture are obtained in increased concentration in the vapour, while less volatile components remain in greater concentration in the liquid residue. Distillation columns may be designed using trays, structured packing, or random dumped packing. Due to restricted access, for small columns below about 750 mm diameter, random dumped packing is preferred.
However, methanol recovered from a kraft pulping process has several unique characteristics that inhibit separation by distillation.
Typically, significant quantities of dimethyl disulphide are present in the crude methanol produced during the kraft pulping process. The presence of an azeotrope between methanol and dimethyl disulphide requires that the methanol content in the SOG be no higher than approximately 40 wt % to ensure separation. Control of the foul condensate steam stripping system, in terms of both the quantity and quality of SOG produced, can reduce the impact of azeotropes of dimethyl disulphide. Many existing stripping systems include a reflux condenser integrated with the multiple effect evaporators; see for example U.S. Pat. No. 4,137,134 to Suominen et al., U.S. Pat. No. 3,807,479 to Brannland et al., and U.S. Pat. No. 5,830,314 to Mattsson. Unfortunately, in this arrangement, control of the stripping system may be compromised because any fluctuations in evaporator operation will ripple through the stripping system, unpredictably affecting SOG quantity and quality.
Further, contaminants including ionizable sulphur compounds such as hydrogen sulphide and methyl mercaptan are produced during the pulping process. These compounds can dissociate under certain conditions, making them all but impossible to remove from SOG by simple distillation. As can be seen in FIG. 1, hydrogen sulphide (H.sub.2S) begins to dissociate at a pH above about 6, while methyl mercaptan (MM) begins to dissociate at a pH above about 9. In their dissociated form, these compounds do not exert a vapour pressure and therefore can not be removed by distillation. Controlling the pH of the liquid phase in the distillation column is therefore an effective way to remove these compounds in a distillation process.
As condensed SOG typically has a pH of about 9 to 10, an acid, such as sulphuric acid, may be metered to the appropriate distillation column to lower the pH in the system. However, the acid cannot simply be added to the liquid feed to the column as it will react with any ammonia present in the system, producing ammonium sulphate. This is known as fouling the column and is to be avoided. U.S. Pat. No. 5,989,394 to Johansson et al. describes a process in which an acidifier is introduced to a stripping column above the admission point of the liquid being purified, or alternatively is added to the liquid feed directly. However, Johansson is concerned with producing a relatively purified condensate stream, rather than removal and high level purification of methanol from the liquid feed stream and does not seem to be concerned with fouling the column.
It is therefore an object of the invention to provide a method and apparatus to recover and purify methanol stripped from a foul gas stream that overcomes the foregoing deficiencies.
In particular, it is an object of the invention to provide a method and apparatus to recover and purify methanol to a high degree, allowing methanol to be used within a kraft pulping process and to allow excess methanol to be sold, rather than destroyed.
These and other objects of the invention will be appreciated by reference to the summary of the invention and to the detailed description of the preferred embodiment that follow.